The invention relates to an article which includes a bi-colored electroplated metallic coating, and process for its manufacture.
A previous patent application (PCT/IL97/00158, claiming priority from Israel Patent Application No. 118281, filed May 15, 1996), of the present Applicant Company, described and claimed articles and a process for making the articles, the latter including a colored electroplated metallic coating comprising both nickel and zinc, on an underplate of copper, brass, bright nickel or matt nickel, supported on a metallic or plastic substrate, the variation of color of the electroplated coating being due to factors other than variation in the identity of ions in the electrolyte, namely, electroplating parameters. Examination of the colored coatings obtained in this process showed that, while they possessed subjectively a particular color, they were analytically mixtures of different colors.
The electrodeposition of nickel on metal substrates such as steel, copper and brass, is widely used in industry in order to meet both decorative and protective requirements for a wide range of goods. The properties provided by an electrodeposited nickel surface, for engineering applications, are generally adhesion, and corrosion- and wear-resistance, hardness and ductility, while for consumer applications the same qualities are relevant, and additionally the appearance of the surface becomes of great importance as part of the decorative value of the products.
The appearance of an electrodeposited nickel coating is usually described in terms of properties such as brightness, reflectivity, tarnish resistance, smoothness, texture and so forth. For esthetic reasons, the color of the coating is also of importance, especially for consumer applications, but the possibilities for imparting intrinsic color to electrodeposited nickel are very limited.
While aluminum may be provided with an oxide film coating which imparts excellent corrosion- and wear-resistance, by an electrolytic process in which aluminum constitutes the anodexe2x80x94xe2x80x9canodizingxe2x80x9dxe2x80x94and while such a coating may be successfully colored, such a technique is not applicable to nickel.
In painting technology, it is known to provide surfaces with pigmented polymeric coatings, in order to obtain articles with a colored finish, but of course the surface is not metallic, and thus cannot for example be selected to be a mirror, matt, full-bright or semi-bright finish. Moreover, the manufacturing process then requires an additional coating-pigmenting step, which it would be desirable to avoid, if this were possible.
It is also known to provide colored metallic finishes on (usually bright) electrodeposited nickel with a restricted range of colors. Thus, various hues and shades of gold can be deposited in this manner from gold cyanide electrolyte, and silver can be plated from cyanide electrolyte from a dissolving silver anode. Similarly, a dark gray-blue finish can be imparted to nickel by electrodeposited ruthenium. Such metallic finishes suffer from the following drawbacks: (a) the color range is limited to golds, silvers and gray-blues; (b) the high price of the coloring component makes such processes expensive, and in case stripping is required this would also be expensive; (c) plating from cyanide electrolytes is neither user-friendly not environment-friendly; (d) each color requires its own special electrolyte, so that the plating bath must be changed in order to change the color.
In an attempt to meet in particular the limitation of the narrow range of obtainable colors, a number of formulations have been developed for coloring metal surfaces electrolytically or by dipping. By way of example, a solution of lead acetate, sodium thiosulfate and acetic acid can produce a blue color on electrodeposited nickel; a solution of potassium chlorate, and copper and nickel sulfates can produce brown colors on brass and copper; and a solution of copper sulfate containing acetic acid and glycerol, in addition to ammonium, sodium and zinc chlorides, produces the socalled tiffany green on brass or nickel, by repeated immersion and drying of the articles in question. Production of such single colors is unlikely to be economical, and it should also be noted that similarly to the previously-mentioned overplating techniques using gold, silver or ruthenium, these colors each require particular process conditions and often exotic electrolytes or dipping solutions, so that the plating conditions and the bath must be changed in order to change the color, which features of course add to the difficulties of carrying out operations which are commercially viable. An additional problem in such cases is that the obtainable colors and hues are sensitive to slight changes in plating parameters, so that the results may depend more on the operator""s skill, than on a particular formulation and plating conditions.
Another approach to solving the problem of the lack of variety of colors available by simply overplating nickel, has been the electrophoretic technique, which involves the deposition of pigment particles in the micronic size range from a pigment suspension in an electroplating bath. Although this technique does provide a variety of colors in the articles thus produced, at the same time the finishes lack the brilliance of nickel-plated articles and are tarnish-like, semi-bright colors. As we have seen in various known techniques, here too, each color requires its special coloring bath, and changing the color means changing the bath. Moreover, stripping of the color is not practical, so that if the finished article is defective in color or appearance, the defect cannot be repaired.
Although not answering consumer demand for a variety of colors, electrodeposition on a metal cathode of a black coating known as xe2x80x9cblack oxidexe2x80x9d or xe2x80x9cblack nickelxe2x80x9d, is also commercially available, and affords a range from light gray to black anthracite. Black nickel is usually plated onto a brass or nickel base, or onto steel provided with an intermediate layer of zinc, copper or nickel. A variety of electroplating conditions and electrolyte formulations for such purposes have been described in the art, but the formulations usually contain zinc, nickel and sulfur, in thiosulfate. These formulations, generally termed xe2x80x9coxidizing liquidxe2x80x9d are available in the market, in concentrated liquid form. According to U.S. Pat. Nos. 4,861,441 and 5,011,744, black nickel coatings of excellent quality are said to be obtainable in presence of a strongly oxidizing anion, and cations of Zn and a xe2x80x9ccoloring metalxe2x80x9d i.e. Fe, Co, Ni, Cr, Sn or Cu, at a pH of 1-4, a current density of 5-100 A/dm2 and a current quantity of 20-200 coulombs/dm2. Somewhat similar are processes for obtaining a black electrodeposited coating, described in U.S. Pat. Nos. 4,968,391 and 5,023,146, in which the bath contains additionally a sulfur compound such as a thiocyanate or a thiosulfate, and the preferred current density is 1-50 A/dm2. Also described in the literature is a process for obtaining black nickel electroplated coatings from a bath containing Ni, Zn and ammonium cations and thiocyanate anions, at a pH of from 3.5 to 6.0, and a cathode current density of 0.15-0.2 A/dm2 (Dennis, J. K. and Such T. E., Nickel and Chromium Plating, 2nd Edition, Butterworth, 1986). W. Schwartz, in Plating and Surface Finishing, June 1982, pages 26-29, describes inter alia formulations for electroplating systems, in order to obtain platings of black chromium, nickel or nickel/molybdenum, or (gray) arsenic.
A phenomenon related to the problem of providing electrodeposited colored metallic surfaces is that of light interference in submicronic/micronic electroplated films, in which the color depends on film thickness. For example, cuprous oxide changes its color from an initial violet through blue, green, yellow, orange and red, due to the interference phenomenon, as the film thickness increases (see e.g. Solomon, H., Isserlis, G. and Averil, A. F., xe2x80x9cProtective and Decorative Coatings for Metalsxe2x80x9d, Finishing Publications Ltd., USA, 1978). However, this phenomenon is not commercially viable because of the unreliability of the desired color, since the slightest changes in electroplating parameters or physical variation in the metal surface, leads to an even more dramatic change, in color or hue, of the electroplated film.
H. Keping et al. in Metal Finishing, June 1996, pages 97-99, described a process in which nickel-plated mild steel was passivated cathodically in a molybdate/phosphate electrolyte, to give blue-purple, gold, green and grass-green coatings, depending on the plating time, each of the mentioned colors being of greater thickness than the one mentioned beforehand.
To the best of the present inventors"" knowledge, the known art (including Keping et al.) describes only electroplated metallic coatings, each having, visually, no more than a single color. The entire contents of the above-mentioned patents and literature references are incorporated by reference herein.
A primary object of the invention is to provide a visually bi-colored electroplated coating on bright or matt nickel as underplate, and a process for the preparation thereof.
Another object of the invention is to provide a visually bi-colored electroplated coating, and a process for the preparation thereof as just recited, wherein the two colors of the visually bi-colored coating can be to some extent varied and predetermined, by selecting process parameters.
Still another object of the invention is to provide a visually bi-colored electroplated coating as aforesaid, and a process for the preparation thereof, wherein the coating has a lustrous brilliant appearance similar to a high level conventional bright or matt electroplated nickel coating.
Yet another object of the invention is to provide a visually bi-colored electroplated coating as aforesaid, and a process for the preparation thereof, wherein the ingredients of the electrolytes used are neither more expensive nor more hazardous than those used conventionally for nickel electroplating.
Yet a further object of the invention is to provide a visually bi-colored electroplated coating as aforesaid, and a process for the preparation thereof, wherein the coloring process is stable, in that acceptable variation of colors can be assured by corresponding variation within a reliable range of process parameters.
Another very important object of the invention is to provide a visually bi-colored electroplated coating as aforesaid, and a process for the preparation thereof, wherein various colors and hues of the colored coating can be produced using the same bath and the same electrolyte solution, by selecting the process parameters exclusively.
By the expression xe2x80x9cvisually bi-colored coatingxe2x80x9d and similar expressions in the present specification and claims, there is to be understood a coating which, when applied to an article including a planar surface, possesses a first visual color when such surface is viewed from a particular angle, and which when viewed from a different angle can be seen to possess a second color different from the first color. It should be noted that the combination of the two colors in the visually bi-colored coating may be a complementary combination of colors, but, as illustrated in the Examples, the invention is not limited thereto. However, since the invention naturally includes articles having a plurality of surfaces disposed at various angles with respect to the viewer, in such cases the articles will appear to be reflecting two different colors.
Other objects of the invention will be apparent from the description which follows.
In one aspect, the present invention provides an article, which may be planar or non-planar, and which includes a bi-colored metallic coating electrodeposited directly on a metallic cathode such that where the article comprises a substantially planar surface, said coating possesses a visual first color when viewed from a first angle to said surface and it possesses a visual second and different color when viewed at a second angle to said surface.
In another aspect, the present invention provides a process for manufacturing an article as defined in the preceding paragraph, which process includes the step of electrodepositing said coating on a suitable metallic cathode from an electrolyte bath which comprises ions selected from the group consisting of molybdenum(VI)-containing ions and (Ni(II)xe2x88x92+Zn(II))xe2x88x92 containing ions, wherein the parameters ionic concentration, pH, bath temperature current density and current quantity are selected so that a bi-colored coating is obtained, subject to the condition that a current density is applied to said underplate as cathode within the range of 0.005 to 0.5, preferably 0.0075 to 0.25 A/dm2.
The terms xe2x80x9celectroplatedxe2x80x9d, xe2x80x9celectroplatingxe2x80x9d and similar terms have their normal meaning in the art and thus exclude, for example, other electrical processes such as electrophoresis.
It is a particular and distinctive feature of the present invention that differently colored bi-colored coatings are obtainable while maintaining the identity of the chemical ingredients in the electroplating bath, in any embodiment of the invention using a particular combination of ions. Thus, according to the invention, selection of the colors of the bi-colored coating of the invention, where the electrolyte includes, for example, molybdenum(VI)-containing ions and (PO4)3xe2x88x92 ions is not determined by adding or subtracting ingredients in the electroplating step, but is rather determined by variation of the parameters: ionic concentration, pH, bath temperature current density and current quantity.
It will accordingly be apparent that the present invention is distinct from the prior art in which gold and silver cyanides can provide, respectively, only gold and silver coatings; where the presence of ruthenium in the bath will give only blue-gray coatings; from so-called xe2x80x9ccoloredxe2x80x9d coatings which are in practice black nickel coatings; from a combination of bath ingredients which gives only the so-called xe2x80x9ctiffany greenxe2x80x9d colored coating, from a different combination of ingredients which gives only a blue coating and from yet a different combination which gives only a brown coating. It will be apparent also that the present invention is distinct from the invention of our previous patent application PCT/IL97/00158. Moreover, the present invention achieves for the first time commercially viable electrodeposited bi-colored metallic coatings. While the present invention is not considered to be limited by any theory, it is possible that the variation in colors of the electrodeposited bi-colored metallic coating and the difference in color between the two visualized colors, is connected on the one hand with the phenomenon of light interference, and on the other hand with viewing different faces of crystalline electrodeposited metal. Presuming this to be so, then the invention for the first time combines the phenomena of light interference, according to which the color of the coating is related to its thickness and the nature of the electrodeposited crystalline metal.